Development of hard-to-cook defect in cowpeas: role of pectin methylesterase.
Liu K. | Phillips R.D. | Hung Y.C.
Development of the hard-to-cook (HTC) defect in cowpeas has been shown to consist of two stages: an increase in cation-uptake capacity followed by binding of divalent cations. Activation of pectin methylesterase (PME) leading to demethylation of cell wall pectin was believed to be responsible for the rust-stage development. To confirm this hypothesis, a headspace gas chromatographic method was used to monitor changes in free methanol, PME activity, and methoxy content in cowpeas during HTC-inducing treatments heat incubation and/or CaCl2 soaking) and adverse storage (30 degrees C/64% relative humidity). Results showed that PME activity decreased progressively with incubation temperature and time. The difference in PME activity between the control and aged seeds was insignificant, while the difference in methanol released was significant. The degree of pectin methylation of the cell wall remained unchanged in both treatment- and storage-induced seeds. It was concluded that an increase in cation-uptake capacity during HTC development is not due to PME action or pectin demethylation, implying inadequacy of the cations-pectin egg-box model to explain the HTC defect.
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